Display subscription name on the admin subscription page

What does this MR do and why?

On /admin/subscription, the Subscription details card shows a license ID that customers can't match to anything in the Customers Portal, so they open support tickets just to confirm which subscription their instance runs. This adds a Subscription name row (the A-S000xxxxx value) right below the ID, as agreed in the issue.

The value already ships inside every CustomersDot-issued license (License#subscription_name reads it from the license restrictions), so this only adds a subscriptionName GraphQL field on CurrentLicense and renders it. Old manually generated licenses can lack the value; the row then shows - like other empty fields.

Two small notes: the label uses sentence case (Subscription name) per Pajamas, though the issue wrote it in title case; and the fragment field carries @gl_introduced(version: "19.3.0") so pre-19.3 backends during rolling deploys return null instead of erroring.

Resolves #477312 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Activate a cloud license on your instance (or any license whose restrictions include subscription_name).
2. Visit /admin/subscription as an admin.
3. The Subscription details card shows Subscription name: A-S000xxxxx below the ID row.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.